How do you measure a rifles rate of twist?

In order to measure the length it takes for a bullet to spin a full rotation, you can use a cleaning jag. Make a mark on the end of the rod and insert it. When the mark has rotated a full 360°, mark the rod flush with the barrel and measure the distance between the two marks. That is your twist rate ratio. So if it takes 10 inches to turn 360°, then your twist rate is 1:10.

There are also special tools that attach to cleaning rods for measuring a rifles twist rate.